The invention relates generally to semiconductor device manufacturing and, more particularly, to photolithographically forming plural features using different illumination conditions.
Semiconductor devices having smaller and smaller features are approaching a limit in which such features may be formed by conventional photolithography methods. For example, conventional photolithography methods produce vias in an insulating layer, as illustrated FIGS. 1A and 1B. A positive photoresist layer 3 is formed over an insulating layer 1 in which it is desired to form a first and second via. A plurality of regions 5A, 5B, 5C and 7 in the photoresist layer 3 are simultaneously exposed to actinic light 8 through openings 11A, 11B, 11C and 13 in a single mask or reticle 9, as illustrated in FIG. 1A. The terms mask and reticle are used interchangeably, with the term reticle often applied to a mask used in step and repeat exposure systems. The exposed regions 5A, 5B, 5C and 7 are then developed and removed, while unexposed regions 6 remain as shown in FIG. 1B. A gas or liquid etching medium is then supplied through the openings 5A, 5B, 5C and 7 in the photoresist layer 3 to etch vias 15A, 15B, 15C and 17 in layer 1, as illustrated in FIG. 1C.
However, as the density of the vias has increased and the size of the vias has decreased, diffraction effects often result in patterning errors. These errors result in via size or location that deviate from the desired size or location.
Furthermore, only certain values of exposure dose and defocus are allowed to form an exposed region in the photoresist layer within the allowed design parameters. A set of allowed exposure dose and defocus values that may be used in exposing a particular region is referred to as a process window.
It has been suggested that the process window 101 for exposing dense regions in a photoresist layer differs from the process window 103 for exposing isolated regions in the photoresist layer, as shown in FIG. 2A. See A. R. Newreueher and C. A. Mack, SPIE Short Course Note SC102, “Optical Lithography Modeling”, Feb. 27, 2000. For large features to be exposed, there is a large overlap 105 between the windows 101 and 103, as shown in FIG. 2A. Therefore, in order to expose both dense and isolated regions, the corresponding value of exposure dose and defocus must be located in the overlap region 105 between the process windows 101 and 103. For large features to be exposed, it is relatively easy to select appropriate values of exposure dose and defocus that fall into the large overlap region 105, as shown in FIG. 2A.
However, the overlap region 205 of process windows for exposing dense 201 and isolated 203 regions in a photoresist layer is relatively small for exposed features having a small size, as shown in FIG. 2B. Region 207 of process window 201 and region 209 of process window 203 outside the overlap region 205 cannot be used when exposing both dense and isolated regions. Therefore, it is difficult to select the exposure conditions which are optimum for both small dense and isolated regions because the process window overlap 205 for exposing such regions is also small.